Piezo actuator comprising a structured external electrode

ABSTRACT

An electrical component includes a base having ceramic layers and internal electrodes between at least some of the ceramic layers. The electrical component also includes an external electrode on a face of the base, which contacts at least some of the internal electrodes. The external electrode includes a layer having at least one local minimum.

TECHNICAL FIELD

This patent application describes an electrical multi-layer component,in particular a peizo actuator. This patent application also describes amethod for producing such an electrical multi-layer component.

BACKGROUND

Piezo actuators are known that have a base body, with a stack ofstratified ceramic layers, and internal electrodes lying between them.The internal electrodes are made from a mixture of silver and palladium.The ceramic layers contain a ceramic based on lead zirconium titanate,which has a piezoelectric effect because of its ferroelectricproperties. Because of the piezoelectric effect, the ceramic expandswhen electrical potential is present, so that it is possible to makeactuators from such a multi-layer ceramic.

Also present in the known piezo actuators are outer electrodes, whichare applied continuously to one lateral face of the base body andcontact the internal electrodes.

In order to reduce the costs of producing the piezo actuators, an effortis made to replace the material of the internal electrodes and thematerial of the outer electrode with copper. In known piezo actuators,the outer electrode has the form of a continuous layer. This form of theouter electrode is not suitable for outer electrodes made of copper.Under thermal demands, which occur, for example, when contact elementsare soldered onto the outer electrode, with a continuous layer there isa shearing force between the outer electrode and the base body of thepiezo actuator, which leads to damage to the boundary layer between theouter electrode and the base body.

This damage to the boundary layer is accompanied by a reduction of theeffective bonding area between the outer electrode and the base body.

A piezo actuator produced in this manner is subjected, in the course ofits use, to a multitude of mechanical strains, which in turn lead toshearing forces between the base body and the outer electrode. Becauseof the relatively high strength of the copper electrode, theseadditional loads result in the damage that arises when the thermal loadspreads across the surface of the boundary layer and leads to detachmentof the outer electrode. Thus the piezo actuator fails and can no longerbe used.

SUMMARY

Described herein an electrical multi-layer components and a method forproducing it, where the danger of detachment of the outer electrode isreduced.

An electrical multi-layer component is described that includes a basebody. The base body contains a stack of stratified ceramic layers, andinternal electrodes lying between them. An outer electrode is placed onone lateral face of the base body, for contacting internal electrodes.

The outer electrode has the form of a layer in which at least oneindentation is provided.

At the place of the indentation, the thickness of the layer can bereduced. That results in an intended tear point at this location, atwhich the tensile and compressive strength of the layer or outerelectrode is reduced. As a result, even relatively small shearing forcesof the outer electrode against the base body are sufficient to produce atear through the layer at the intended tear point. As a result, theshearing load acting on the entire layer can be significantly reduced,since the tensile and compressive forces that produce the shearing loadalong the layer or along the surface of the base body can only add upover a correspondingly shorted distance.

In particular, the layer can have a local minimum thickness at thelocation of the indentations.

In just the same way, however, it is also to design the indentation inthe outer electrode in such a way that the outer electrode isinterrupted at the location of the indentation. As a result, in a sensea tear is provided already when the outer electrode is produced, so thatit is inherently impossible for great shear forces to result fromaddition of tensile and compressive forces over long distances.

The outer electrodes can have areas with essentially constant layerthickness, which has the benefit that outer electrode can be applied viaa screen-printing present.

In an embodiment of the electrical multi-layer component, the outerelectrode contains copper, enabling the material costs for the componentto be reduced.

Furthermore, it is beneficial if the ceramic layers arepiezoelectrically active. This is beneficial because the electricalmulti-layer component can be used as a piezo actuator.

A piezoelectric effect is obtained, for example, through the use of aceramic based on lead zirconate titanate.

The indentations can run in the form of troughs, with the troughsextending along a longitudinal axis. The projection of theselongitudinal axes on the lateral face of the stack of stratified ceramiclayers or internal electrodes on the side of the outer electrodeintersects the internal electrodes at an angle. That increases thechances that every internal electrode that runs along the correspondingouter surface of the stack can be contacted.

This means that the angle α takes on a value that 0 and 180°.

It is possible to advantageously arrange a plurality of indentations atequal distances. This has the benefit that the outer electrode isdivided uniformly into a plurality of areas, which means a correspondingreduction in the maximum occurring shearing force.

In addition, a plurality of indentations can be distributed uniformlyover the layer.

Furthermore, ti is also possible to arrange a plurality of indentationsin such a way that they form a periodically recurring structure (forexample a rhombus or square).

In order to promote the function of the indentation as an intended tearpoint, it is advantageous for the minimum layer thickness at the pointof the indentation to be a maximum of 75% of the layer thickness in thearea where the layer thickness is essentially constant.

The outer electrode can be applied in the form of a screen-printingpaste that contains copper powder.

Applying the outer electrode in the form of a screen-printing paste hasthe benefit that by using appropriate screen-printing masks; a reliableand uniform arrangement of indentations can be achieved.

When applying the outer electrode in the form of a screen printing pasteor by a screen printing process, the indentations may have a minimumwidth of 200 μm, since otherwise it may not be possible to guarantee adefined indentation on the basis of the consistency of the paste or themesh number used. Smaller indentations can be realized only withincreased effort and expense, due to the low viscosity of thescreen-printing paste normally used. It would be conceivable, however,to produce indentations with smaller lateral extents by using a morestructurally viscous or more thixotropic screen-printing paste.

In addition, a method for producing an electrical multi-layer componentis specified that includes the following:

a) Production of a base body with a stack of stratified ceramic layersand internal electrodes lying between them, and with an outer electrodeplaced on a lateral face of the base body for contacting internalelectrodes, which has the form of a layer and in which at least oneindentation is provided.

b) Contacting of the outer electrode with a contact element whileexerting a shearing load between the outer electrode and the lateralface of the base body.

Normally materials are used for the ceramic layers and the outerelectrode whose thermal expansion coefficients differ from each other.

The method for producing the multi-layer electrical component has theadvantage that in spite of exerting the shearing load, because of theintended tear points or already existing tears formed by theindentation, the danger of detachment of the outer electrode is reduced.

Despite the use of materials to their particular function with normallydiffering thermal expansion coefficients for the outer electrode andceramic layers, the foregoing method allows the contacting of the outerelectrode with a contact element to take place by soldering.

When copper is used for the outer electrode and when a PZT ceramic isused for the ceramic layers of the stack, the possible exists ofperforming the soldering at a temperature >200° C., which allows the useof correspondingly high-melting solders. Such high-melting solders havethe advantage that they permit use of the electronic components, forexample a piezo actuator, at temperatures of up to 150° C., such asoccur, for example, on the combustion engine of a motor vehicle.

Embodiments will now be explained in greater detail with reference tocorresponding figures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of an electric multi-layer component inschematic cross-section.

FIG. 2 shows an additional embodiment of an outer electrode.

FIG. 3 shows different profiles of shearing forces with a continuous andan interrupted outer electrode.

FIG. 4 shows indentations in the form of troughs.

FIG. 5 shows indentations arranged along a regular grid.

FIG. 6 shows an example of an electrical multi-layer components with anintermediate layer formed at the edge of the base body.

DETAILED DESCRIPTION

FIG. 1 shows a piezo actuator, with a base body 1 that includes a stack1 a of stratified ceramic layers 2 and internal electrodes 3 lyingbetween them. The piezo actuator or the base body 1 is shown lying down.The base and cover surface of stack 1 a are thus located on the rightand left sides of FIG. 1, respectively. The internal electrodes 3 reachalternately to the upper edge and to the lower edge of stack 1 a. Alongpassive zones 13 only every second internal electrode 3 is present, sothat in this zone only a very small deflection of the piezo actuatorresults when an electrical potential is applied between the internalelectrodes 3 emerging from the lower end of stack 1 a and those emergingfrom the upper end of stack 1 a.

An outer electrode 5 is placed on the upper lateral face 4 of the basebody 1, for contacting internal electrodes 3. The outer electrode 5 hasindentations 6. It can be seen from the detailed view of FIG. 1 that theindentations 6 are in the form of local minima of the layer thatdetermines the form of the outer electrode. Within the indentation 6,the layer thickness d is a minimum of d_(min). In addition, the outerelectrode 5 has areas of essentially constant layer thickness d, whichmeans that in these areas the layer thickness d varies by less than 10%.It can also be seen from the detail of FIG. 1 that the indentation 6 hasa width b, which should not fall below a certain minimum dimension whenthe outer electrode is applied by a screen printing process, sinceotherwise the normally used screen printing processes have to bespecially adapted, for example by using a more structurally viscousscreen printing paste.

The minimum layer thickness d_(min) in the area of indentation 6 shouldbe not more than 75% of the layer thickness d of the outer electrode 5,since it may not otherwise be possible to guarantee the intended tearpoint.

The minimum layer thickness d_(mm) in the area of indentation 6 shouldbe not more than 75% of the layer thickness d of the outer electrode 5,since it may not otherwise be possible to guarantee the intended tearpoint.

An additional embodiment is shown in FIG. 2. There, the outer electrode5 on the lateral face 4 of the base body 1 is interrupted at the pointof the indentation 6. This means that the layer thickness d is zero inthe area of the indentation 6. The embodiment has the advantage thatunder a thermal load, for example, the outer electrode 5 does not haveto tear only at the location of intended tear point, but that theinterruptions already exist and the maximum occurring shearing force isalready inherently reduced (see also the explanation for FIG. 3).

FIG. 3 explains the effect that interrupting an external electrode 3 hason the shearing forces that occur in the case for example of a deflectedpiezo actuator. To this end, the base body 1 of a piezo actuator isshown schematically in FIG. 3. The deflection of the piezo actuator isrepresented schematically by double arrows. On the top side of base body1, a continuous outer electrode 5 a is shown. On the bottom of the piezoactuator or of base body 1, an outer electrode 5 b interrupted by theindentation 6 is shown schematically in cross section. If one considersthe same tensile stress for the upper outer electrode 5 a and the lowerouter electrode 5 b (indicated by the two double arrows), this producesthe profile shown below FIG. 3 for the qualitative profile. Curve 10describes the profile of the shear stress S as a function of thelongitudinal coordinate z of base body 1 for the continuous outerelectrode 5 a. On the outer hand, curve 11 describes the profile of theshear stress S as a function of the longitudinal coordinate z of basebody 1 for the interrupted outer electrode 5 b.

Since the tensile or compressive stresses add up over a longer distancein the case of the outer electrode 5 a, the grater maximum shear loadalso arise for the outer electrode 5 a, identified in FIG. 3 as S₂. Inthe case of the interrupted outer electrode 5 b, the tensile orcompressive stress can only add up over a shorter distance, which leadsto the fact that a smaller maximum shearing load S₁ results, as can beseen from the profile of curve 11. This lower maximum shear stressresults in a reduction of the danger that the outer electrode 5 b willtear off under a mechanical load, for example by pulling on wiresconnected to the outer electrode 5 b, or also by a plurality ofdeflections of the piezo actuator.

A favorable minimum width b is around 200 μm. A typical layer thicknessd producible by screen printing processes is around 15–25 μm.

FIG. 4 shows indentations 6 in an outer electrode 5 in the form oftroughs running along a longitudinal axis 7. FIG. 4 is a top view of anexample of an outer electrode 5. In addition, FIG. 6 shows lines 8 thatsymbolize the profile of the internal electrodes 3 corresponding toFIG. 1. The longitudinal axes 7 run essentially parallel to each otherand intersect the lines 8 at an angle α, which is other than 0° and180°. The result is that every second internal electrode, represented bylines 8, is contacted by areas 14 with an essentially constant layerthickness d of the outer electrode 5. This makes it possible to ensurethat every second internal electrode 3 is contacted by the outerelectrode 5 or by areas 14 with an essentially constant layer thicknessof the outer electrode 5.

FIG. 5 shows a top view of another exemplary embodiment for a structuredouter electrode 5. Here indentations in the form of circles areprovided, which form a square grid.

When the outer electrode 5 is applied in the form of a screen printingpaste, it is usual to use a screen printing paste that contains glassfrit. This glass frit is used to improve the mechanical bonding of theouter electrode 5 to the base body 1. Depending on the glass frit usedand depending on the ceramic used, or depending on the setting ofadditional process parameters, it can occur that the glass frit forms anintermediate layer 9 lying between the outer electrode 5 and the edge ofthe base body 1, which intermediate layer 9 is interrupted in the areaof the internal electrodes 3. When considering the problem of theshearing off or separation of the outer electrode 5, the intermediatelayer 9 of glass frit must be allocated to the base body 1, as can beseen from FIG. 6. Detachment processes may take place between the outerelectrode 5 and the intermediate layer 9.

FIG. 6 also shows two contact elements 12, which may be in the form, forexample, of thin wires, and which are connected to the outer electrode 5by soldering. When a PZT ceramic is used for the ceramic layers that hasa thermal expansion coefficient of 1.5–2.0×10⁻⁶ m/mk, and when using anouter electrode 5 of copper with a thermal expansion coefficient of19×10⁻⁶ m/mk, providing the indentations 6 results in the possibility ofperforming the soldering of the contact elements 12 at a temperature ofaround 300° C., which permits the use of high-melt solders, for examplePb-based solders.

For the glass frit it is possible, for example, to use a combinationthat contains lead oxide, silicon oxide, boric oxide and possiblyadditional components.

The apparatus and methods described herein are not limited to use inpiezo actuators, but may be applied to all multi-layer components, forexample also to capacitors.

1. An electrical component comprising: a base comprising: ceramiclayers, and internal electrodes among at least some of the ceramiclayers; and an external electrode on a face of the base, the externalelectrode contacting at least some of the internal electrodes, theexternal electrode comprising a layer that has an indentation, wherein athickness of the layer at the indentation is a local minimum thickness.2. The electrical component of claim 1, wherein the external electrodecomprises areas having a layer thickness that is substantially constant.3. The electrical component of claim 1, wherein the external electrodecomprises copper.
 4. The electrical components of claim 1, wherein theceramic layers are piezoelectrically active.
 5. The electrical componentof claim 1, wherein the layer of the external electrode comprises pluralindentations, the plural indentations being disposed at an anglerelative to the face of the base.
 6. The electrical component of claim1, wherein the layer of the external electrode comprises pluralindentations, the plural indentations being spaced apart from oneanother at substantially equal distances.
 7. The electrical componentsof claim 1, wherein the layer of the external electrode comprisesplurality indentations, the plural indentations being distributedsubstantially uniformly over the external electrode.
 8. The electricalcomponent of claim 1, wherein the layer of the external electrodecomprises plural indentations, the plural indentations forming aperiodically recurring pattern.
 9. The electrical component of claim 1,wherein the external electrode has a substantially constant layerthickness at area other than the indentation.
 10. The electricalcomponent of claim 9, wherein the indentation has a maximum of 75% ofthe substantially constant layer thickness.
 11. The electrical componentof claim 1, wherein the external electrode is formed from a screenprocessing paste containing copper powder.
 12. The electrical componentof claim 1, wherein the indentation has a width of at least 200 μm. 13.A method for producing an electrical component, comprising: producing abase, the base comprising: ceramic layer, and internal electrodes amongat least some of the ceramic layers, a face of the base comprising anexternal electrode that contacts at least some internal electrodes, theexternal electrode comprising a layer that has an indentation, wherein athickness of the layer at the indentation is a local minimum thickness:establishing contact between the external electrode and a contactelement; and exerting a shearing force between the external electrodeand the face of the base containing the external electrode.
 14. Themethod of a claim 13, wherein the external electrode and the ceramiclayers comprise materials with differing thermal expansion coefficients;and wherein soldering is used to establish contact between the externalelectrode and the contact element.
 15. The method of claim 14, whereinthe external electrode comprises copper, the ceramic layers comprise aPZT ceramic, and the method further comprises: attaching wires to theexternal electrode by soldering at a temperature that is greater than200° C.
 16. The method of claim 13, wherein the shearing force isexerted while contact is being established.
 17. An electrical componentcomprising: ceramic layers; electrodes among at least some of theceramic layers, the ceramic layers and the electrode layers togetherforming a stack having a first surface and a second surface, theelectrode layers comprising alternating first electrodes and secondelectrodes, the first electrodes extending to the first surface but notto the second surface, the second electrodes extending to the secondsurface but not to the first surface; and an external electrode on thefirst surface, the external electrode contacting the first electrodes,and the external electrode comprising a layer having one or more localindentations, wherein a thickness of the layer at an indentation is alocal minimum thickness.
 18. The electrical component of claim 17,wherein the stack comprises passive zones adjacent to the first surfaceand the second surface.
 19. The electrical component of claim 17,wherein each of the one or more indentations in the layer comprising theexternal electrode is a local minimum thickness.
 20. The electricalcomponent of claim 19, wherein each of the one or more indentations areat least 25% less thick than a remainder of the layer comprising theexternal electrode.
 21. The electrical component of claim 17, furthercomprising one or more wires soldered to the external electrode.
 22. Theelectrical component of claim 17, wherein the one or more indentationsform troughs that are at an angle relative to the first surface of thestack.
 23. The electrical component of claim 22, wherein the troughsform substantially regular patterns on the first surface of the stack.